Ion exchange membranes are in wide use as a membrane for cell (e.g. solid polymer fuel cell, redox flow cell or zinc-bromine cell), a membrane for dialysis, etc. Solid polymer fuel cell, which uses an ion exchange membrane as an electrolyte, is an electricity generation system in which a fuel and an oxidant are supplied into a cell continuously, they are reacted, and the resulting chemical energy is taken out as an electric power; and it is an electricity generation system which is clean and highly efficient. In recent years, the solid polymer fuel cell has increased its importance for uses in automobile, household and portable device because it can be operated at low temperatures and can be produced in a small size.
The solid polymer fuel cell has inside a solid polymer membrane which functions as an electrolyte, and the solid polymer membrane has, at each side, a bonded gas diffusion electrode having a catalyst loaded thereon. This cell works as a fuel cell when a fuel composed of hydrogen gas or an aqueous alcohol (e.g. methanol) solution is fed into a chamber (fuel chamber) in which one of the gas diffusion electrodes is present, an oxygen-containing gas as an oxidant (e.g. oxygen or air) is fed into a chamber in which the other gas diffusion electrode is present, and an external circuit is connected to the two gas diffusion electrodes. Of fuel cells, direct-liquid fuel cell which uses an aqueous alcohol (e.g. methanol) solution as a fuel, is easy to handle because the fuel is a liquid. Further, this fuel cell uses a fuel of low cost. Therefore, the direct liquid fuel cell is expected as an electric source of relatively small output used especially in portable devices.
The fundamental structure of direct-liquid fuel cell is shown in FIG. 3. In FIG. 3, 1a and 1b are each a partition wall of cell. The cell partition walls 1a and 1b are formed at the both sides of a solid polymer electrolyte membrane 300 used as a diaphragm so as to sandwich the solid polymer electrolyte membrane 300. 2 is a fuel passage formed in the inner wall of one cell partition wall 1a. 3 is an oxidant gas passage formed in the inner wall of other cell partition wall 1b. 4 is a diffusion electrode of fuel chamber side. 5 is a gas diffusion electrode of oxidant chamber side.
In this direct-liquid fuel cell, when a liquid fuel such as alcohol or the like is supplied into a fuel chamber 7, proton (hydrogen ion) and electron are generated from the liquid fuel by the catalyst possessed by the fuel chamber side diffusion electrode 4. The proton passes through the inside of solid polymer electrolyte membrane 300 and reaches an oxidant chamber 8, where the proton reacts with oxygen gas or with the oxygen in air, generating water. The electron generated at the fuel chamber side diffusion electrode 4 passes through an external circuit (not shown) and is sent to the oxidant chamber side gas diffusion electrode 5. At this time, the external circuit is provided with an electric energy.
In the direct-liquid fuel cell having the above-mentioned structure, there is ordinarily used a cation-exchange membrane as the above-mentioned membrane. The cation-exchange membrane is required to have properties of small electric resistance, high physical strength and low permeability to alcohol or the like used as a fuel. For example, when the membrane has a high permeability to alcohol, the alcohol in the fuel chamber diffuses into the oxidant chamber side, resulting in a reduced cell output.
As the cation-exchange membrane used as a membrane for fuel cell, there is, for example, one in which a porous membrane made of a polyolefin resin or a fluoroplastic is used as a base material. This cation-exchange membrane is produced by a method which comprises filling, in the pores of the base material, a polymerizable composition composed of a polymerizable monomer having a functional group into which a cation-exchange group can be introduced and a crosslinkable polymerizable monomer, polymerizing the polymerizable composition, and introducing a cation exchange group into the functional group into which a cation-exchange group can be introduced. The resulting membrane composed of a crosslinked polymer having a cation-exchange group can be produced at a relatively low cost, is small in electric resistance, is low in permeability to liquid fuel, and is low in the swelling and deformation caused by the fuel; therefore, the membrane is preferred (for example, Patent Literatures 1 and 2).    Patent Literature 1: JP2001-135328 A    Patent Literature 2: JP1999-310649 A